Colloidal Silica for Sapphire and Glass Polishing: CMP Beyond the Silicon Wafer
A technical guide to applying colloidal silica CMP slurry beyond silicon—covering the tribochemical polishing mechanism for sapphire (Al₂O₃), process parameter optimization for LED and power device substrates, display glass final polishing, precision optical components, and hard disk drive substrates.
The semiconductor industry is colloidal silica’s largest and most technologically demanding market, but it is far from its only application. The same properties that make colloidal silica ideal for silicon and oxide CMP—nanometer-scale particle uniformity, tribochemical surface reactivity, chemical tunability, and process flexibility—also make it the preferred abrasive for precision polishing of sapphire substrates for LED and power electronics, large-format display glass panels, precision optical components, and hard disk drive substrates.
In each of these applications, colloidal silica achieves surface quality results that conventional abrasives cannot match, through a fundamentally similar tribochemical mechanism adapted to the specific surface chemistry of each substrate material. This guide covers the science and practice of each major non-silicon application.
1. Colloidal Silica Beyond the Silicon Wafer
The versatility of colloidal silica as a precision polishing abrasive stems from a single material property: the surface silanol groups (Si-OH) on every colloidal SiO₂ particle can form transient chemical bonds not only with SiO₂ surfaces but also with the hydrated surfaces of other oxide materials—Al₂O₃, SiO₂ glass, aluminosilicate glass—enabling the same tribochemical removal mechanism to function across a wide range of substrate chemistries.
The mechanism in all cases is analogous to silicon dioxide CMP: alkaline slurry chemistry attacks the substrate surface to form a softer, hydrated surface layer; the colloidal silica abrasive forms transient bonds with the hydrated layer; relative motion between abrasive and substrate shears surface fragments away; the cycle repeats. What changes between applications is the specific chemistry required to hydrate each substrate, the particle size and pressure parameters needed to optimize MRR vs. surface quality for each material’s hardness and roughness specification, and the pH window dictated by each substrate’s chemical stability.
2. Sapphire: Material Properties and CMP Challenges
Single-crystal sapphire (corundum, α-Al₂O₃) is the dominant substrate material for GaN-based blue and UV LED devices, and is increasingly used for GaN-on-sapphire power transistors (GaN HEMT), micro-LED display backlights, and RF filter substrates. Its dominance is due to excellent thermal stability, transparency to UV and visible light, close lattice match to GaN (a-plane mismatch of only 13.8%), and relatively low cost at 2–4 inch wafer sizes.
However, sapphire’s material properties make it one of the most challenging substrates to polish to semiconductor-grade surface quality:
- Dureza extrema: Mohs hardness 9.0 (quartz = 7.0; silicon = 6.5–7.0; diamond = 10). Only diamond and boron nitride are harder than sapphire among commercially relevant materials.
- High fracture toughness: K₁c ≈ 2.0 MPa·m^0.5, meaning sapphire resists crack propagation and requires high contact stresses to mechanically damage the surface—but once damaged (by too-aggressive abrasive or too-high pressure), the sub-surface damage layer extends micrometers deep.
- Crystal anisotropy: Sapphire’s hexagonal crystal structure means polishing rates and surface reactivity differ between crystal orientations (c-plane vs. r-plane vs. a-plane), complicating process optimization for multi-orientation wafer batches.
- Chemical inertness: Al₂O₃ is resistant to most acids and bases at room temperature, requiring specifically engineered alkaline chemistry to achieve meaningful chemical removal rates.
3. Tribochemical Mechanism for Al₂O₃ Removal
The tribochemical removal mechanism for sapphire with alkaline colloidal silica is analogous to, but chemically distinct from, the mechanism for SiO₂:
- Surface hydration: At pH 10–12, hydroxide ions (OH⁻) attack Al-O bonds at the sapphire surface, forming aluminum hydroxide species (Al(OH)₄⁻ in solution, Al(OH)₃ or AlOOH as a surface layer). This hydration layer has significantly lower hardness and yield strength than the underlying crystalline Al₂O₃.
- Abrasive-surface interaction: Silanol groups on the colloidal SiO₂ abrasive form hydrogen bonds and—under contact pressure—condensation bonds (Si-O-Al) with the aluminum hydroxide surface species. The Si-O-Al bond provides the chemical adhesion needed for efficient load transfer.
- Mechanical shear: Relative motion between the pad and the sapphire wafer shears the bonded aluminum hydroxide fragment from the surface, exposing fresh Al₂O₃ for the next hydration cycle.
- Transport: The removed Al(OH)₃ fragment dissolves in the alkaline slurry (forming Al(OH)₄⁻) and is carried away from the polishing interface by slurry flow.
Higher pH accelerates sapphire removal—but within limits: Increasing pH from 10 to 12 can double or triple the sapphire MRR by accelerating both surface hydration and Al dissolution. However, above pH 12, the slurry becomes corrosive to some tool components (seals, delivery lines) and the pH adjustment chemicals (KOH, NaOH) introduce metallic contamination risk. Most production processes optimize at pH 10.0–11.5.
4. Process Parameters for Sapphire CMP
Sapphire CMP process parameters differ significantly from silicon CMP due to sapphire’s much greater hardness and the different tribochemical mechanism:
| Parámetro | Silicon Final Polish | Sapphire Final Polish | Why Different |
|---|---|---|---|
| Down-force (pressure) | 0.5–1.5 psi | 2–5 psi | Sapphire’s hardness (Mohs 9) requires higher stress to initiate tribochemical contact |
| Table speed | 30–80 rpm | 60–120 rpm | Higher velocity compensates for lower chemical removal rate per contact |
| Pad type | Soft (Suba, Politex) | Semi-hard to hard (IC1000 or Politex depending on step) | Hard pad needed for efficient load transfer on stiff sapphire surface |
| Abrasive D50 | 20–35 nm | 50–100 nm | Larger particles needed for acceptable MRR on harder substrate |
| pH | 10.5–12.0 | 10.0–11.5 | Alkaline chemistry targets Al-O bonds; above 12, tool compatibility issues |
| Typical MRR | 100–400 Å/min | 50–250 Å/min | Lower due to sapphire’s extreme hardness despite higher pressure/velocity |
| Polish time | 120–600 s | 600–3,600 s | Lower MRR requires substantially longer polishing for epi-ready finish |
The most important practical consideration is multi-step process design. Sapphire wafer manufacturing typically involves: (1) slicing from boule and rough grinding (diamond wheel) to remove bulk material; (2) mechanical lapping with Al₂O₃ or SiC abrasive to achieve flatness and remove deep grinding damage; (3) rough CMP with coarser colloidal silica or ceria slurry to remove lapping damage; (4) final CMP with fine colloidal silica (D50 = 50–80 nm) at pH 10–11.5 to achieve the epi-ready surface specification. The step sequence is analogous to silicon wafer polishing but with longer times at each step due to the lower MRR on sapphire.
5. Surface Quality Targets: LED vs Power Device Substrates
Surface quality requirements for sapphire substrates differ by end device application:
- LED (GaN blue/UV epitaxy): Surface roughness <0.3 nm RMS over 5×5 µm² AFM scan; pit density <1,000 cm⁻²; subsurface damage depth <5 nm; no visible scratches at 100× DIC microscopy
- Power GaN (HEMT, Schottky): Surface roughness <0.2 nm RMS; pit density <500 cm⁻²; surface metal contamination <1×10¹⁰ atoms/cm² for Fe, Cu; atomically smooth step-terrace structure preferred for AlGaN/GaN buffer growth
- UV-C LEDs (AlGaN on sapphire): Roughness <0.1 nm RMS; highest specification due to AlN nucleation sensitivity to surface steps and pits; typical process uses D50 = 50–70 nm colloidal silica at pH 10.5–11.0
- Micro-LED and display: Roughness <0.5 nm RMS; LPD <100 at >0.1 µm; lower spec than power device but high wafer-level uniformity (WIWNU ≤5%) is critical for multi-die display applications
6. Display Glass Polishing
Large-format display glass—borosilicate glass for LCD panels, aluminosilicate glass for OLED substrates, and ultra-thin glass for foldable display covers—is polished with colloidal silica to achieve the surface quality required for pixel-level uniformity and optical clarity. As of August 2026, Gen 10.5 LCD panels (2,940 × 3,370 mm substrate size) are in volume production at several Asian display manufacturers, all using colloidal silica-based CMP in the final glass polishing step.
6.1 Polishing Requirements
Display glass polishing requirements include: surface roughness <0.5 nm RMS (visible light scattering from roughness above this threshold degrades display contrast ratio); zero sub-surface crack damage (cracks propagate under thermal cycling and cause panel delamination); and strict particle count on the glass surface after polishing (particles cause pixel-level defects in the TFT or OLED layer deposited on the glass).
6.2 Slurry Formulation for Glass
Display glass polishing uses colloidal silica in the 80–150 nm D50 range at pH 7–10—lower pH than for silicon CMP because borosilicate glass surface chemistry is active at near-neutral pH, and aggressive alkaline conditions can leach boron from the glass surface, altering the surface composition. Solids content is typically 10–20 wt% at the point of use. The tribochemical mechanism is essentially identical to SiO₂ CMP on silicon wafers: OH⁻ ions hydrolyze Si-O-Si bonds in the glass surface, creating a soft gel layer that the colloidal silica abrasive removes by shear.
7. Optical Glass and Precision Optics
Precision optical manufacturing—telescope mirrors, laser optics, semiconductor photomask substrates, and reticle glass blanks—was historically one of the first industries to adopt colloidal silica as a precision polishing abrasive, predating its widespread use in semiconductor CMP by several decades. The extreme surface form accuracy requirements (λ/100 surface figure error for laser cavity mirrors) and sub-0.1 nm RMS roughness specifications for laser optics represent the highest precision demands for any colloidal silica polishing application.
Key process features for precision optical polishing:
- Very fine abrasive: D50 = 20–50 nm for final figuring polish on optical-grade fused silica or borosilicate glass
- Ultra-low solids content: 2–10 wt% working concentration minimizes surface roughness contribution from abrasive-pad-substrate contact dynamics
- pH 8–10: Moderate alkalinity to activate surface hydration without glass composition degradation
- Very long polish times: Hours to days for large telescope mirror blanks (>1 m diameter); this demands exceptional slurry stability (see Section 7 in our stability guide)
- Computer-Controlled Optical Surfacing (CCOS): Colloidal silica is the standard slurry for magnetorheological finishing (MRF) and ion beam figuring support polishing steps in precision optics manufacturing
For photomask substrates (synthetic fused silica blanks for EUV and DUV lithography), colloidal silica is used in the final CMP step to achieve <0.05 nm RMS roughness and ≤0.1 nm peak-to-valley over any 1 mm² area—among the tightest surface quality specifications in any industrial polishing application.
8. Hard Disk Drive Substrate Polishing
Hard disk drives (HDD) use aluminum or glass platters (substrates) that must be polished to extreme flatness and smoothness to enable reliable sub-5 nm head-disk spacing in modern perpendicular magnetic recording drives. The surface roughness requirement (<0.10 nm RMS) is comparable to silicon wafer final polish, while flatness requirements (total thickness variation <2 µm) are tighter than for standard optical flats.
Colloidal silica is used in the final polishing steps for both aluminum (NiP-coated Al alloy) and glass HDD substrates. For aluminum substrates, the slurry must include aluminum corrosion inhibitors (such as benzotriazole or phosphate-based inhibitors) to protect the NiP-coated surface from chemical attack at alkaline pH while still enabling tribochemical removal. For glass substrates, standard alkaline colloidal silica at pH 8–10 achieves the required roughness with no corrosion inhibitor needed.
9. JEEZ CS-100 for Non-Silicon Applications
The JEEZ CS-100 Series (D50 = 90–110 nm, pH 10.0–11.5) is the recommended starting point for sapphire CMP, display glass final polish, and HDD glass substrate polishing. Its particle size and pH range are optimized for the higher-pressure, moderate-roughness-target applications that characterize these non-silicon substrates. For ultra-high-quality optical glass or UV-C sapphire applications requiring roughness below 0.2 nm RMS, the JEEZ CS-60 Series (D50 = 55–70 nm) provides a better roughness-throughput balance.
JEEZ provides application-specific formulation recommendations and process integration support for all non-silicon substrate polishing applications. For the complete colloidal silica technical reference: Colloidal Silica Slurry: The Complete Guide to CMP Applications, Properties, and Selection.
10. Frequently Asked Questions
The effectiveness of colloidal silica on sapphire is not due to the abrasive being harder than the substrate—it isn’t. Instead, it works through the same tribochemical mechanism as silicon CMP: alkaline slurry chemistry (pH 10–12) attacks the sapphire surface to form a thin, softer aluminum hydroxide (Al(OH)₃ or AlOOH) hydration layer with significantly lower hardness than the underlying crystalline Al₂O₃. The colloidal silica abrasive removes this hydrated layer rather than the hard bulk sapphire. This chemical-assistance mechanism enables effective material removal at much lower contact stresses than purely mechanical abrasion of crystalline Al₂O₃ would require.
For the final CMP step targeting GaN epi-ready quality (<0.3 nm RMS, <1,000 pit/cm²), use D50 = 50–80 nm colloidal silica at pH 10.0–11.0 and process pressure of 2–4 psi. For UV-C AlGaN applications requiring tighter roughness (<0.15 nm RMS), use D50 = 50–65 nm with lower pressure (1.5–3 psi) and longer polish time. The rough CMP step before final polish typically uses D50 = 80–150 nm or a mixed ceria/colloidal silica formulation to achieve acceptable throughput on the harder substrate.
The same colloidal silica grade (same D50, same pH) can in principle be used on both substrates, but the process parameters (pressure, velocity, pad type, polish time) must be re-optimized for each substrate. Silicon final polish uses D50 = 20–35 nm at low pressure (0.5–1.5 psi) on soft pads; sapphire CMP uses D50 = 50–100 nm at higher pressure (2–5 psi) on semi-hard pads. Using the fine silicon polish slurry (D50 = 25 nm) on sapphire will produce unacceptably low MRR and very long polish times. Dedicated slurry grades optimized for each substrate are the preferred approach in production.
For borosilicate display glass (LCD-grade), use pH 7–9. Aggressive alkaline conditions (pH >10) can leach boron and alkali metal ions from the glass surface, altering the surface composition and potentially causing haze or corrosion defects. For aluminosilicate glass (OLED/foldable displays, which have higher Al₂O₃ content and greater alkali resistance), pH 9–10.5 is acceptable and provides higher MRR. The pH optimum for any specific glass composition should be verified experimentally, as glass surface reactivity depends strongly on composition.
JEEZ Colloidal Silica for Sapphire, Glass, and Specialty Substrates
Our CS-60 and CS-100 series are used in sapphire CMP, display glass final polish, and optical component polishing worldwide. Contact our application engineering team to discuss your substrate specification and process requirements.
Request Application Guidance →Published by Jizhi Electronic Technology Co., Ltd. (JEEZ) · August 2026. For process-specific advice, contact our application engineering team.
